1. Field of Invention
The present invention relates to an arrangement of a liquid crystal electrooptical device and more particularly to an arrangement of a liquid crystal electro optical device which is capable of quick response.
2. Description of Related Art
There has been conventionally known display devices utilizing liquid crystal typified by the TN type liquid crystal. The TN type liquid crystal electro-optical device operates as follows by adequately selecting the following two states.
There is one state in which liquid crystal material interposed and held between a pair of substrates is spiraled such that the direction of major thereof is turned by 90.degree. from one substrate to the other substrate. There is also another state in which the major axes orient in the direction of the pair of substrates (in the direction vertical to the substrates).
An optical state of light transmitting through the liquid crystal layer is changed by selecting either of the above-mentioned two states to select whether to transmit or not to transmit the light which enters the liquid crystal panel.
In this operation, the liquid crystal molecules are caused to orient in the direction between the substrates (hereinafter this is defined as the first state) by applying electrical potential between a pair of electrodes disposed on the pair of substrates.
When no electrical potential is applied to the pair of electrodes, the liquid crystal molecules are tuned and spiraled by 90.degree. from one substrate to the other substrate (this is defined as the second state) due to the orientation processing means provided on the substrates in advance.
In the operation described above, the change from the second state to the first state is caused by applying electric field from the electrodes disposed on the pair of substrates. That is, it is caused by forcibly orienting the major axes of the liquid crystal molecules by the electric field by counteracting orientation restricting force.
Meanwhile, the change from the first state to the second state is caused by the orientation restricting force generated from the orientation processing means provided on the substrates. That is, the liquid crystal molecules which have been put into the predetermined state by the electric field forcibly by counteracting the orientation restricting force are put into the state in which the direction of the major axes is turned by 90.degree. between the substrates by the orientation restricting force when no electric field is applied. It is noted that the orientation restricting force is caused mainly by inter-molecular force.
There has been also known an arrangement in which active elements typified by a TFT (thin film transistor) are disposed in each pixel in order to carry out the above-mentioned operation more quickly. This arrangement is made such that electric charge applied to the pixel electrode is controlled by the active elements per each pixel. A liquid crystal display having this arrangement is called as an active matrix type liquid crystal display.
When this active matrix type arrangement is adopted, the change from the second state to the first state is caused by the electric field under control of the active elements. However, the change from the first state to the second state is caused still by the orientation restricting force.
In the active matrix type liquid crystal display, although the speed of the change from the second state to the first state may be increased by utilizing the thin film transistor which responds quickly as the active element the speed of the change from the first state to the second state caused by utilizing the orientation restricting force cannot be increased even if the active element is improved.
In the arrangement in which the display is made by switching the two states which the liquid crystal can take, the change from the second state to the first state and that from the first state to the second state are caused in different ways as described above. That is, the electric field is utilized to obtain the first state from the second state and the orientation restricting force is utilized to obtain the second state from the first state.
However, the response speed thereof cannot be increased even if the active elements are improved in the method utilizing the orientation restricting force. It means that there is a certain limit in the response speed in switching the two states. It then becomes a factor for defining an upper limit of characteristics of a liquid crystal shutter which is required to operate at high speed or of a display which is required to display in high definition or at high speed.
Further, unclear or unnatural display is caused by the difference of operating modes (asymmetrical operation) in taking the above-mentioned two states as the display is operated at high speed.
Accordingly, it is an object of the invention disclosed in the presents specification to solve the above-mentioned problem of the method of changing from one state to the other state by applying the electric field and from the other state to one state by the orientation restricting force as the method for realizing the two states, of liquid crystal.
That is, it is an object of the present invention to correct the asymmetry of the changes from a dark state to a bright state and from the bright state to the dark state.
It is another object of the present invention to provide an arrangement for realizing the quick response.